1. Field of the Invention
The present invention relates to an LED backlight, and more particularly to a white LED backlight having improved color reproduction capability, or color rendering capability, and to a liquid crystal display device using it.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2004-158452 discloses a backlight unit in which a refracting member is provided between white LEDs and a light guide plate to refract and redirect light from the white LEDs to emerge in the direction of the light guide plate. Further, Japanese Patent Laid-Open No. 2005-56842 discloses a backlight assembly that uses both white LEDs and single-color LEDs (red LEDs) to achieve enhanced color reproduction capability.
Conventional LED backlights employing white LEDs have an advantage over LED backlights employing three primary color LEDs (i.e., red, green, and blue LEDs) in terms of luminance uniformity and color uniformity, but have lower color reproduction capability.
Specifically, a white LED actually emits apparent white light produced as a result of mixing blue wavelength light from a blue LED and yellow wavelength light emitted by a fluorescent material when the material is excited by that blue wavelength light. Therefore, LED backlights employing while LEDs have poor red color reproduction capability. On the other hand, LED backlights employing three primary color LEDs have high color reproduction capability, since light from each primary color LED has an intensity peak at a respective primary color wavelength. However, it is difficult to mix light from these primary color LEDs so as to produce white light having high color uniformity.
Further, in the case of LED backlights using three primary color LEDs, these LEDs must be disposed in close proximity to mix light from them, which means that each LED is heated by heat from the adjacent LEDs, resulting in reduced luminous efficiency. On the other hand, LED backlights using while LEDs do not have such heat dissipation problems (or do not require a special heat dissipation arrangement), since these white LEDs can be spaced apart from each other.
Further, in the case of side backlights (employing white LEDs), since the white LEDs are disposed adjacent to and along a side of the light guide plate to allow light to exit through the entire light exit surface of the light guide plate, the color of light from the backlights varies depending on the spectral characteristics (or spectroscopic characteristics) of these white LEDs. Further, the apparent white light emitted by the white LEDs has no intensity peak at red wavelengths and hence does not include sufficient red color components, since it is produced by mixing blue wavelength light from a blue LED and yellow wavelength light emitted by a yellow fluorescent material when the material is excited by that blue wavelength light, as described above.
To address this problem, the white LEDs may be used in combination with red LEDs. However, in order to prevent color irregularities, it is necessary to fully mix light from them. (Or, a large number of red LEDs must be employed to attain close LED spacing.)